When an impulse is received at the input of a terminal station from a far-end terminal station an impulse echo is produced in the return path towards the far-end terminal station. The echo signals are currently eliminated by connecting an echo canceller at the terminal stations of the telecommunication system.
In particular, digital echo cancellers are used to provide an echo estimate of the incoming signal, which echo estimate is subtracted from the echo signal produced in the return path. A digital echo canceller essentially comprises a digital filter including a number of delay cells. Such a filter is controlled by adjusting the coefficients of each cell in such a way as to obtain the suitable impulse response which is a fairly exact replica of the echo signal.
An echo impulse appearing in an up link shows a response curve that depends on the delay created by the user terminal circuits. These circuits include a 4-wire circuit directly connected to the up and down links and a 2-wire circuit connected to the user side. Each of said circuits generates a respective delay and produces in the return path echo samples which extend over a relatively long period of time.
The total delay introduced by these circuits, and consequently the number of samples, currently varies from one telephone link to the other. Therefore the echo cancellers used so far in the satellite communication systems are controlled to cover the major part of the echo samples produced both by the 4-wire and 2-wire circuits. By way of consequence, the filters used in these known digital echo cancellers must comprise a great number of delay cells, the control of which requires a great number of filter taps to be adjusted and high computer power and time for performing the required control.